1. Field of the Invention
The present invention relates to a one-dimensional illumination apparatus using an array laser as a light source thereof to emit one-dimensional laser light.
2. Description of the Related Art
A method using a rod lens, a method using a mirror treated by a facet cutting process, a method using a lens array, and the like are known as a method with which one-dimensional laser beams, that is, linear laser beams with a uniform light intensity distribution are obtained. In such a method, a laser light source is used, a wavefront is divided, and the divided wavefronts are superimposed to generate the linear laser beams. However, in any of these methods, fine interference fringes are observed at the focused portion due to spatial coherence inherent in laser light. When a wavefront is divided by a distance shorter than a spatial coherence distance of laser light, divided wavefronts have interference on each other and interference fringes are generated when the divided wavefronts are superimposed.
However, when a wavefront is divided by a distance longer than the spatial coherence distance in actual laser light, since the coherence distance is not so small as compared with a diameter of a beam spot of laser light, it is difficult to obtain a sufficient number of wavefronts. Thus, when the divided wavefronts are superimposed, sufficient uniformity of light intensity may not be obtained. Also, even when the wavefront is divided by a distance longer than the spatial coherence distance, spatial coherence may not be removed completely and interference still remains.
Japanese Unexamined Patent Application Publication No. 2006-49656 and Japanese Unexamined Patent Application Publication No. 2004-12757 propose methods of reducing interference fringes by combining divided wavefronts (or wavefronts to be divided) with an optical path difference longer than a time coherence distance.
FIGS. 1 and 2 of the accompanying drawings show schematic diagrams of optical systems of the above-described methods according to the related art. As shown in FIG. 1, laser light Lo emitted from a light source (not shown) passes through an optical path difference generation member 59, which generates an optical path difference. Subsequently, a wavefront thereof is divided by a pair of cylindrical lens arrays 52a and 52b, each having a function of dividing the wavefront and superimposing divided wavefronts. Afterward, an object 54 is illuminated with the laser light passing through a focusing lens 53. As shown in FIG. 2, the optical path difference generation member 59 can be located between the cylindrical lens arrays 52a and 52b. 